This invention relates to an ink-printable transfer film on which an image may be printed.
Transfer materials have been available for many years and typically consist of a polymeric or wax film which is supported on a carrier sheet. An image is printed on the film before the film is transferred from the carrier sheet to a substrate usually by applying pressure or heat to the back of the carrier sheet.
In French patent application published as FR 2 715 607, a method is described for decorating an substrate with an image which has first been printed on a transfer material. A digital image from a conventional video camera is printed using an ink jet printer onto the transfer material which comprises a plastic support and a heat-sensitive adhesive coating onto which the ink is printed. The printed side of the transfer material is placed in contact with the substrate and heat applied to activate the support. The plastic support can be removed once the adhesion between the coating and the substrate is greater than that between the coating and the plastic support.
FR 2 715 607 does not disclose the composition of the coating which accepts the printing ink. However, known heat-sealable coatings are essentially continuous films deposited from a polymer solution in organic solvent or water, or from a dispersion of polymer in water with emulsifiers.
There are particular problems associated with using ink jet or bubble jet printers to print images on transfer films. The inks of jet printers are solutions of dyes in solvent which, in use, are sprayed through nozzles onto the surface to be printed. In industrial jet printers, volatile organic solvents are employed and quick drying of the ink is achieved by evaporation of the solvent. However, with desktop jet printers, non-volatile solvents such as water and glycol mixtures must be used to avoid premature drying of the ink and clogging of the spray nozzles. With non-volatile solvents, the ink is held by absorption into the substrate instead of evaporation. The non-volatile solvents work well with absorbent substrates such as paper, but tend to sit on the surface of non-porous substrates such as the known heat-sealable coatings. With certain filmic substrates, such as overhead projection transparencies, it is possible to overcome the absorbency problem by coating the film with water soluble or water-swellable polymers prior to printing. However, these polymers are not sufficiently thermoplastic to be transferable by heat to another substrate, nor are they sufficiently waterproof to produce permanent graphics.
According to the present invention, there is provided a transfer film for transferring an ink comprising at least one liquid component, the film comprising a porous matrix of particles of a heat activatable adhesive bound together by an absorber, the absorber being at least partly soluble in the said liquid component within the porous matrix, and the absorber preferably being within the pores of the matrix.
Preferably the porous matrix of adhesive particles is carried by a carrier film.
Although the film is described as comprising a matrix of particles of adhesive with an absorber within the matrix, it will be appreciated that in isolation the structure could also be described as comprising particles of adhesive within a matrix formed by the absorber. The absorber has the double function of binding the matrix of heat activatable adhesive and at least partially absorbing the liquid component of the ink.
The transfer material can be used with inks that comprise a pigment or mixture of pigments and a liquid component that acts as a vehicle for the pigment. Examples of vehicular components are non volatile solvents such as water and glycol. The absorber should be at least partially soluble in the vehicle of the ink with which the transfer film is to be used. The absorber component is at least partially dissolved by the vehicular component of the ink and on application of the ink a viscous solution of absorber and vehicular component is formed. The viscous solution holds the ink pigment preventing spreading of the ink and blurring of the transferred image formed by the ink.
Preferably the absorber forms a viscous solution of viscosity at least 1500 cp preferably at least 1800 cp, when partially dissolved in the ink vehicle.
Here, the term non-volatile solvent is used to describe the vehicular component of the ink but is not meant to imply that the solvent does not evaporate at all when the ink is incorporated on to the surface of the substrate. In the case of components such as glycol and water xe2x80x9cnon-volatilexe2x80x9d implies that the retention of the dye component of the ink on the substrate is primarily by absorption of the solvent into the substrate and subsequent deposition of the dye component on to the substrate. This is in contrast to other types of ink where the dye is deposited onto the substrate by evaporation of a volatile solvent.
The matrix of particles of adhesive has a porous structure into which ink can be drawn by capillary action. The film thus provides an absorption surface for the vehicular components used in inks and overcomes the tendency of the ink to form droplets on the film surface. Such droplet formation will result in the ink drying on the surface of the film with consequent reduction of the surface tack required to transfer the film to the chosen substrate; this problem is avoided by transfer films according to the present invention.
The absorber also functions as a binder. This results in the transfer forming an almost continuous layer of ink receiving material. A printed image may be applied to the transfer sheet and absorbed by substantially the whole surface area of the transfer sheet, with only very small particles of heat activatable adhesive being present in the surface layer. The small heat activatable adhesive particle size (for example a particle of heat activatable adhesive may have a diameter of between 0.08 and 3 microns, for example 0.1 micron) means that the heat activatable adhesive will have a minimal effect on the resolution of the printed image. Thus transfer films according to the invention give rise to very high resolution images comparable in quality to images produced on the best photographic quality inkjet media currently available.
Preferably the absorber is a heat softenable material.
Preferably, the absorber is a water soluble or hydrophilic organic polymer and the transfer film is used with an ink that contains water as a vehicular component. The inclusion of a water-soluble absorber within the porous matrix of the transfer film moderates the porosity of the film, reducing the spread of ink in the film, and increases overall strength. The water-soluble absorber component holds the ink by interacting with the non volatile ink components (for example water or glycol) and hence allows high definition images to be printed by preventing ink drawn into the film by capillary action from migrating noticeably beyond the point of initial contact.
Preferably the water soluble absorber is acrylate copolymer, cellulose ether, polyvinyl pyrollidone or a combination of two or more of these.
Preferably the ink printable transfer film is supported on a carrier. Preferably the absorber binds the transfer film to the carrier.
Preferably the carrier is a resin coated paper or polymer film, for example polyethylene coated paper, or polyester or polypropylene film. The choice of carrier depends on the surface finish desired.
Preferably the transfer further comprises a wetting agent.
Preferably, the transfer material further comprises an alkaline component such as sodium hydroxide or ammonia solution that stabilizes the transfer material formulation when it is being made and applied to the carrier.
The heat activatable adhesive in the ink printable transfer film has a heat activation temperature also known as the film formation temperature.
In one embodiment of the invention, the ratio of absorber to heat activatable adhesive is chosen so that when the film is placed in contact with a substrate and heated the heat activatable adhesive may form a layer over only a part of the surface of the transfer film. Preferably, the ratio of absorber to heat activatable adhesive is above 1:5 (i.e. greater than 0.2). Preferably, the ratio of absorber to heat activatable adhesive is 1:1.5 (i.e. 2 to 3, or 0.67).
In another embodiment of the invention, the ratio of absorber to heat activatable adhesive is chosen so that when the film is placed in contact with a substrate and heated the heat activatable adhesive forms a layer of adhesive over substantially the whole surface of the transfer film. The transfer film is thus attached to a substrate over substantially the whole surface of the transfer film. Preferably, the ratio of absorber to heat activatable adhesive is 1:9.4.
Preferably the absorber is a softenable absorber to allow greater interaction between the adhesive of the transfer film and the substrate.
Preferably, the heat-activatable adhesive is selected such that the minimum film forming temperature of the transfer sheet is in excess of about 20 degrees C above a typical ambient workshop temperature. More preferably, the heat-activatable adhesive is selected such that the minimum film forming temperature is below about 125xc2x0 C. More preferably the heat-activatable adhesive is selected such that the minimum film forming temperature is below about 75xc2x0 C. Preferably, the film of heat activatable adhesive forms over a temperature interval. Preferably the interval is within the range 750 and 125xc2x0 C. The particles will not then have a tendency to coalesce when stored in the workshop, but will start to form a continuous film when the adhesive is fully activated, when heat is applied to transfer the coating from the carrier sheet to the chosen substrate.
The transfer film may further comprise a stabilizer such as Tinuvin 213, which will prevent fading of the dye and retard degradation of polymers in the transfer film.
The transfer film may further comprise a thickener, for example a polyacrylate such as Latekoll D. Such thickeners aid in production of the transfer film by thickening the formulation mix so it can be applied more easily to a carrier material.
The transfer material may further comprise a weak acid such as boric acid. Boric acid acidifies the finished transfer material which aids in complexation and absorption of components of printing inks, thereby preventing migration of the ink and increasing resolution of the final image. Boric acid, being a weak acid, will not interfere with the alkaline components of the transfer material sufficiently to prevent stabilization of the transfer formulation when it is being made and applied to a carrier.